The technical field of the present invention is that of materials for producing packaging. More particularly, the present invention relates to a coated film intended for packaging an article requiring an extended shelf life, of the agar culture medium type.
Many polymer films, capable of being used for packaging products, are present on the market. Mention may be made, in particular, of films for food use such as films made of polyamide (PA), polyethylene terephthalate (PET) or polyvinyl chloride (PVC). Other films such as those made of low-density polyethylene (LDPE), used for bundling (grouping together of products) or palletization, may be stretchable and shrinkable. Certain films are also composed of high-density polyethylene (HDPE) or else polypropylene (PP). Finally, in certain particular applications, the films may be composed of a complex of various polymers mentioned above.
The choice of the material to be used to produce the film depends generally on the properties that it is desired for the film to have. These properties may be of a mechanical nature, such as impact strength or creep resistance, stiffness, stretchability, tearability, flexural strength, etc. Other properties are of a physical nature, such as the capacity to transfer water (water vapor barrier, impermeability), gas transfer, ability to withstand sterilization, freezing or else being microwaved, etc. Finally, other properties may be of an esthetic nature, such as the visual appearance (transparency, gloss, etc.) or the feel.
When the interest lies more particularly in the field of in vitro diagnosis, which is the field of activity of the Applicant, and in particular in films used for bagging culture media, it is observed that the materials customarily used are materials having a low barrier property, characterized by a high water vapor permeability (>120 g/m2×24 hours). Such a material is, for example, cellophane. This material has the advantage of allowing the water contained in ready-to-use agar culture media to evaporate and pass through the film. This then prevents too much condensation inside the bag constituted by the cellophane film. However, the main drawback is that, since water vapor passes through the film, the moisture content inside is very low, leading to greater, and therefore premature, drying of the culture medium. Therefore, the shelf life of the product is thereby affected.
Other materials also used for bagging culture media have, themselves, a high barrier property, characterized by a low water vapor permeability (<5 g/m2×24 hours). This low water vapor permeability does not make it possible to remove the significant condensation that is formed in the dishes of ready-to-use agar media, especially after the media have been poured. It follows that this water remains in the bag until it is opened by the end user, generating splashes and soiling, which is unacceptable. Such products are for example polyolefins, such as PP or PE. Polyolefins are widely used as packaging material. However, the processes for obtaining such materials mean that the latter have very low water vapor permeability. Furthermore, materials are also found that comprise two laminated films, such as PA+PE films, intended to accentuate their water vapor barrier property. Thus materials of this type have water vapor permeability values below about ten grams/m2×24 hours.
Document EP-0 062 800 describes a polyamide film, optionally covered on one of its faces with a polyvinylidene chloride resin. The polyamide used is a biaxially-oriented Nylon-6/Nylon-6,6 copolymer. This type of polymer enables the film thus obtained to have shrinkability properties. It is thus used for packaging food products.
Although this film may have properties suitable for the packaging of foodstuffs, it does not have the properties expected in the field of in vitro diagnosis for the packaging of culture media. This is because, on the one hand, the shrinkability properties are not desired and, on the other hand, the film as described does not display particular water vapor permeability values in line with expectations.
Document FR-1.386.921 describes compositions based on vinylidene chloride for coating various supports. In particular, in example 9, it describes a vinyl chloride/vinylidene chloride copolymer used to coat a polyamide film on one if its faces.
The film thus obtained has an excessively low water vapor permeability value. In other words, this film is almost impermeable. Such a film is therefore absolutely not suitable for use for the packaging of ready-to-use agar media, for the reasons stated above.
The same problem is faced with the polyamide films described in document FR-1.314.025 or else in document GB-1 310 933.